In the transmissions known from the practice which are capable of making the starting operation possible without special starting elements, such as a fluid clutch, a converter, or a separate starting clutch, the engaging elements that slip during the starting operation are exposed to high thermal loads. The clutch has to absorb the complete power loss or frictional energy particularly when the starting operation is effected via a single engaging element or a single clutch. This high load requires a correspondingly sturdy design of the clutch, which disadvantageously entails the need of larger space, greater weight of the engaging elements and elevated production costs.
DE 38 12 327 has disclosed a process for adjusting a double-clutch transmission and a double-clutch transmission of a motor vehicle. The double-clutch has one input shaft and one output shaft which can be interconnected by spur-toothed gear pairs selectively meshable. One first clutch and one second clutch are provided in which the halves of the first clutch are non-rotatably connected with the input shaft and the halves of the second clutch with a shaft or a hollow shaft surrounding the shaft. Engaging means serve to actuate the clutches in order to drive in one gear the force flow between input and output shafts via the clutch, the shaft, the gearshift sleeves and a first set of gear pairs or, in an adjoining gear via the second clutch, the hollow shaft, gearshift sleeves and a second set of gear pairs. When starting the motor vehicle, one appertaining clutch and one appertaining gearshift sleeve of one gear, and the outer clutch and one gearshift sleeve of another gear allocated to the other clutch are simultaneously actuated. Then when the output speed of the clutch of one of the gears reaches the input speed thereof, the force flow is interrupted in the gear.
For the friction or friction work of the starting clutch or starting clutches for a double-clutch transmission in which both input clutches are used for starting, this known process or this double-clutch transmission constitutes a possibility of reducing the thermal load. The proposed process does offer the advantage that the considerable energy produced on the friction surfaces when starting, which is converted to heat in the clutches, has not been absorbed by only one engaging element but is distributed between two engaging elements. But the distribution takes place uniformly and independently of the temporary operating condition of the clutch so that it can result in an overheating and thermal overstress of a clutch which, for example, already has greater wear phenomena than the other clutch.
In addition, it is disadvantageous that the process can be used only in double-clutch transmissions in which the input shaft and the output shaft are interconnected by spur-toothed and selectively meshable gear pairs.
It is further disadvantageous that the process is inadequate or only limitedly adequate mainly for transmissions with a serial arrangement of the engaging elements.
Another considerable disadvantage of this process is that, prior to terminating the starting operation, one clutch that must not be engaged for the subsequent travel has to be disengaged. By opening the engaging element or the clutch, or by actuating the gearshift sleeve, the positive engagement for the gear ratio after starting is thus undesirably interrupted.